The shape of a Brillouin gain spectrum (which will be referred to hereinafter as “BGS”) in which the gain resulting from the Brillouin scattering that has occurred in an optical fiber changes according to the temperature and distortion at the position where the gain has occurred due to the Brillouin scattering in the optical fiber. A technique is known, which measures temperature and distortion by utilizing the change.
In the following non-patent document 1, a technique is described, which measures the distortion of an optical fiber using the BOCDA (Brillouin Optical Correlation Domain Analysis). The BOCDA is a technique to cause a gain to occur at a position where both phases of probe light and pumping light caused to enter both ends of a measuring optical fiber in opposite directions match with each other in the measuring optical fiber. In the technique described in the following non-patent document 1, a gain is caused to occur at each position of the measuring optical fiber by changing the phase difference between probe light and pumping light and thus the distribution of distortion of the measuring optical fiber is found by measuring the BGS of each gain.
Non-patent document 1: Kazuo HOTATE, et al., “Simplified System of Fiber Brillouin Optical Correlation Domain Analysis for Distributed Strain Sensing”, The 16-th Optical Fiber Sensor International Conference (OFS-16), October, 2003, We2-3, p. 290-293.